\Ac*cre"tion\, n. [L. accretio, fr. accrescere to
increase. Cf. {Crescent}, {Increase}, {Accrue}.]
1. The act of increasing by natural growth; esp. the increase
of organic bodies by the internal accession of parts;
organic growth. --Arbuthnot.
2. The act of increasing, or the matter added, by an
accession of parts externally; an extraneous addition; as,
an accretion of earth.
A mineral . . . augments not by grown, but by
accretion. --Owen.
To strip off all the subordinate parts of his as a
later accretion. --Sir G. C.
Lewis.
3. Concretion; coherence of separate particles; as, the
accretion of particles so as to form a solid mass.
4. A growing together of parts naturally separate, as of the
fingers toes. --Dana.
5. (Law)
(a) The adhering of property to something else, by which
the owner of one thing becomes possessed of a right to
another; generally, gain of land by the washing up of
sand or sail from the sea or a river, or by a gradual
recession of the water from the usual watermark.
(b) Gain to an heir or legatee, failure of a coheir to the
same succession, or a co-legatee of the same thing, to
take his share. --Wharton. Kent.

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Biology Dictionary

Definition:

A gradual growth through accumulation of more units. For example, the collection of cells on a filter would be an accretion of the cells.

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Legal Dictionary

Definition:

The increase or accumulation of land by natural causes, as out of a lake or river.